Modern electronic wire and cable typically includes insulated electrical conductors, such as copper wire, bound together in a common protective jacket or sheath. The conductors are insulated from each other by coating them with an insulating material using an extrusion process, such as pressure extrusion or tube/sleeve extrusion. Under accepted industry standards, individual conductors are allowed to include a predetermined amount of defects or pin-holes in the insulation, which are measured by “spark” tests. Such imperfections are essentially unavoidable during the fabrication of the individual conductors and can result in “hi-pot” (high potential) test failures in cabled conductors if the current traveling through those conductors arcs with shield tape disposed around the conductors.
Shield tape is typically applied around cabled conductors to shield the conductors from the undesired effects of external influences, such as electromagnetic radiation. A variety of different constructions of shield tape have been applied around conductors in a number of different configurations to shield the conductors from such effects. Shield tape constructions generally include thin metallic foil layers, such as aluminum, laminated with a layer of insulating film, such as polyester, that form opposing sides of the shield tape. The layer of insulating film is provided to add strength and durability to the shield tape as well as to insulate the aluminum layer. A non-insulated grounding wire, or “drain” wire, is disposed on the aluminum side of the shield tape in electrical contact therewith to provide a low resistance electrical connection, or drain, to ground from substantially any point along the shield tape.
Shield tape is typically applied either helically wound around the conductors or longitudinally wrapped, i.e., “cigarette” wrapped, around the conductors. In both applications, the longitudinal edges of the shield tape generally must overlap one another by a relatively large amount, such as 25%, to prevent the shield from leaking radiation. The shield tape may be applied around the conductors either with the aluminum side facing outward away from the conductors and the drain wire disposed on the outside of the shield tape between the shield tape and the jacket or with the aluminum side facing inward toward the conductors and the drain wire disposed between the shield tape and the conductors. There are significant problems, however, with those conventional configurations of the shield tape and drain wire.
Shield tape is generally helically wound around the conductors to improve the flexibility of the cable. Helically wound shield tape, however, is prone to loosening and kinking at the overlapping edges when it is flexed during use or when drawn through various types of conduits during installation. Loosening and kinking of the shield tape may create spiral slots around the circumference of the shield that radiate interference rather than inductively coupling interference. The interference may radiate as much as 360° around the shield. Although it is also possible for slots to appear at the overlapping edges in cigarette wrapped shielding, those slots will be longitudinal and will radiate interference less effectively because they radiate interference only in the plane of the longitudinal slot. In addition, helically wound shield tape has a greater tendency to conform to the conductors than cigarette wrapped shield tape and is therefore less geometrically stable and more likely to form slots in the shielding.
Helically wound shield tape may be applied to the conductors during the cabling/stranding of the conductors. When shield tape is helically wound around the conductors during cabling/stranding, the shield tape is drawn over the conductors as the conductors rotate, or twist, together. To allow sufficient overlap of the shield tape edges and to ensure that the shield tape is tightly wound around the conductors, the twist lay length of the conductors must be short. Not only do short lay lengths require slower cabling/stranding speeds, they also require a greater amount of conductor material to make the same length of cable, which in turn results in a larger signal delay through the conductors. To apply helically wound shield tape around conductors with larger lay lengths with sufficient overlap and tightness, additional equipment must be used to rotate the shield tape around the conductors at a slower rate than the conductors are being twisted together. This extra machinery can be cost prohibitive.
Helically wound shield tape may also be applied to the conductors subsequent to the cabling/stranding of the conductors. When shield tape is helically wound around the conductors subsequent to cabling/stranding, the shield tape may be applied with sufficient overlap and tightness around the conductors irrespective of the conductors' lay length. This process, however, requires that the conductors be collected on a reel after cabling/stranding and then paid off that reel into separate machinery that applies the shield tape, which requires additional man hours and multiple staging areas and is overall less efficient and more expensive than applying shield tape during cabling/stranding.
As discussed above, the drain wire may be applied between the shield tape and the jacket or between the shield tape and the conductors for either helically wound or cigarette wrapped conductors, depending on the side of the shield tape that faces the conductors. When the shield tape is applied with the aluminum side facing downward toward the conductors, the drain wire must be disposed between the conductors and the shield tape. To prevent the drain wire and/or shield tape from arcing with defects in the conductors and to prevent the drain wire from damaging the insulation on the conductors, a barrier layer of insulating material is typically applied around the conductors so that the aluminum side of the shield tape is in contact with the barrier layer and the drain wire is disposed therebetween. Applying an additional layer of insulating material around the conductors, however, requires additional material and machinery and greatly adds to the costs of manufacturing the cable.
In view of at least the above-identified problems, it is preferable to manufacture shielded cable by applying shield tape around the conductors in a cigarette wrapped configuration with the aluminum side of the shield tape facing outward away from the conductors. Even this configuration, however, creates several problems. For example, the dies used to fold the shield tape suffer significant wear when the aluminum side of the shield tape faces outward away from the conductors because the aluminum side of the shield tape is thereby placed in frictional contact with the dies as the shield tape moves through the dies. Although those dies are typically coated with a protective material to protect against excessive wear, the shield tape will still wear through the protective material when drawn through the dies at higher speeds. And, although pre-lubricated shield tape may be purchased, such shield tape can be cost prohibitive.
In addition, when the aluminum side of the shield tape faces outward away from the conductors, the drain wire must be disposed on the outside of the shield tape so the drain wire will be in electrical contact with the shield tape. Placing the drain wire outside the shield tape, however, creates a bulge in the otherwise flat surface of shield tape surrounding the conductors. If the cable jacket is pressure extruded over the assembly, the jacket will fill in around the drain wire and cause a groove to form on the inside of the jacket and/or a ridge to form on the outside of the jacket. And, if the jacket is tube/sleeve extruded over the assembly, the cable jacket will stretch around the drain wire and cause a ridge to form on the outside of the jacket. A groove on the inside of the jacket compromises the integrity of the cable by creating a thinner portion of jacket extending the length of the jacket, and a ridge on the outside of the jacket will compromise the integrity of the cable by not only adversely affecting the aesthetics of the cable, but also by making it more difficult to draw the cable through various types of conduits during installation.
Accordingly, there is a need for a device of and method for manufacturing shielded cable that allows the conductors to be shielded in a cigarette wrapped configuration, allows the drain wire to be on the outside of the shield tape without forming a ridge, and minimizes the amount of leakage in the shield. Further, there is a need to manufacture such a cable without causing excessive wear to the folding dies and while reducing the amount of additional cable material, man hours, work space and machinery required to shield the cable.